Oscillator system



Jan. 17, 1956 E. o. KEIZER ETAL 2,731,565

oscILLAToR SYSTEM Filed nec. 29, 1951 GIP/0 0F 7065 2, EUEENE EL KEIZERAND MHRLINEKHDEERZ l BY ATTORNEY United States Patent i oscnrsronsrsrsivt Eugene Q. Keizer and Marlin G. Kroger, Princeton, ll. 3.,

assignors to Radio Corporation of America, a corporation of DelawareApplication December 29, 1951, Serial No. 21S-4,194

6 Claires. {C 2513-36) This invention relates to oscillators and, inparticular, to a dual oscillator system wherein the oscillators arealternately' operative.

ln existing systems of this type, separate switching circuits areprovided to switch the oscillators in and out of operation. Wherehigh-speed electronic switching is desired, a circuit such as a bistablemultivibrator or ilipiiop is frequently utilized. The use of suchadditional switching circuits necessitates more complicated circuitryand results in higher equipment cost and greater maintedifficulties.

an object of the present invention to provide a dual oscil tor circuitwhich permits switching of oscillators y. .Lat additional switchingcircuits.

A further obiect of the present invention is to provide inexpensive dualoscillator system which has two stable, oscillatory states.

Still further object or" the present invention is to provide an improveddual oscillator system the operation of which may be shifted from oneoscillator to the other tion are accomplished by utilizing two electrondischarge tubes. rl'he anodes of the tubes are connected through sparateplate load resistors to B+. The grid of each l e is connected to itsanode through a iirst capacitor, a .ed tank circuit and a secondcapacitor. Each tank circuit comprises an inductor and a capacitor whichhave such vaines that the circuits will oscillate at differentfrequencies. The grid of each tube is also connected through a grid loadresistor and through a resistor-capacitor delay network to a common gridbias resistor. With these connections, the rst tube and its associatedcircuit e. ents makes up a first oscillator and the second tube itsassov 'ated elements rnalte up a second oscillator. When the iirstoscillator is operating, it builds up a negative bias at its grid andacross the common biasing resister of such a value that the secondoscillator is cut oli. Conduction is switched from the iirst oscillatorcircuit to the second oscillator circuit by means of negative triggeringpulses which are applied to the common biasing resistor.

The novel features of the invention, as well as the invention itself,both as to its organization and method of operation, will leest beunderstood from the following description, when read in connection withthe accompartying drawings in which:

Figure l shows a circuit diagram of an embodiment of the invention,

Patented Jan. 17, 1956 Figure 2 shows the Voltage wave forms which occurat various points during the operation of the circuit shown in Figure 1.

Since the circuit shown in Figure 1 is a symmetrical dual oscillatorsystem, corresponding circuit elements in each half of the system willbe given the same number followed by a distinguishing letter. A letter awill refer to all elements in a first section and the letter b willrefer to elements in the second section. Oscillator circuits A and B areof the Class C Hartley oscillator type, although a Coipitts or Meissneroscillator or any other type could be used, provided that suitableprovision is made to insure that it operates as a Class C oscillator.The various types of oscillators which could be used under Class Cconditions Vare shown on page 480, Radio Engineers Handbook by Terman,iirst edition published by McGraw-Hill Book Company, 1943. Class Coperation is required, as will be fully explained below, to provide anegative cut-off bias.

Each oscillator A and B comprises an electron discharge tube lila, ib.The anode 11a, 1lb of each tube lila, ldb is connected to B+ through aplate load resistor la, 12b. The tank circuit 19a, 19in of eachoscillator comprises a capacitor 13a, 18!) and an inductor 26a, 2%. Oneend of each tank circuit is coupled through a D.-C. blocking capacitor14a, i415 to the anode of the related tube. rl`he other end of the tankcircuit is connected to the grid 13a, i317 of the tube through a biascapacitor 36a, leb. A tap is made intermediate the ends of each inductor29a, 2%!) and is connected to ground. A grid load resistor 22a, 22b isconnected to each grid. A parallel resistor-capacitor delay network 2&1,28k is connected to each grid load resistor. A common grid bias resistor29 is connected to the junction P of the two delay networks. A negativetrigger pulse input 3% is provided and coupled to the ends of the commongrid bias resistor 29.

The grid bias of each oscillator has two critical levels.

The iirst of these levels is suicient to prevent any tank.

excitation through the tube and consequently causes the termination ofany existing oscillation. The second is less negative than the above butstill suiiiciently negative to prevent initial current flow so that anidle oscillator will be prevented from going into oscillation.Intermediate the two levels above is a range where the grid bias issufiiciently negative to cut oif normal iiow, while allowing positivepeaks from the oscillator tank circuit to cause conduction.

When the first oscillator A is operating, the grid voltage of its tubeis in the intermediate range referred to above where the bias isdeveloped by the flow of grid current during the positive peaks of theoscillator tank circuit. The grid current which passes through the gridfrom the iirst oscillator tube also passes through its grid loadresistor 22a, the resistor in its delay network 25a and the common gridbias resistor 29. The voltage developed thereby across the coinrnon gridbias resistor 29, while less than the bias developed at the grid of theiirst oscillator tube, is sufficiently negative to prevent initialcurrent ow through the second oscillator B. Thus, this second biasacross the common grid bias resistor corresponds to the second of thecritical bias levels referred to above. As long as the first oscillatorremains in operation,` thev second oscillator` is keptidle. ln asimilarfashion, when second oscillator is operating, it develops a grid bias onthe grid of its associated tube and across the common grid bias resistorwhich is suiicient to prevent operation of the rst oscillator.

The above discussion relates to the steadystate, bistablecharacteristics of the circuit shown in Figure l. The triggering orswitching operation will now 'oe considered. Let it be assumed that thefirst oscillator A is in operation. A negative triggering puise, such asis shown in Figure 2, is applied to the junction P of the first andsecond delay networks 2da and 23h, and is of sufficient magnitude to cutoit theoscillationof the tirst oscillator. Thus, the bias which appearsat the grid of the first oscillator tube a at the beginning of thetrigger pulse reaches the first critical level referred to above wheretank circuit excitation is prevented and oscillation terminated. As aresult of the negative triggering pulse, the irst oscillator becomesnon-conductive and ceases to supply charging current to the biascapacitor 16a connected to the oscillator tube grid. Consequently, thebias capacitorilrz begins to discharge through the grid load resistor22a, and the common grid biasing resistor 29. The time-constant of thedischarge circuit for the bias capacitor is made small relative to thetriggering pulse duration such that the negative bias developed acrossthe common grid bias resistor 29 falls rapidly. The bias voltage appliedto the second oscillator tube grid through its delay-circuit resistor26!) and grid-load resistor 22h becomes less negative than the cut-otvalue of the second oscillator tube and allows the initiation of currentflow and oscillation through to the second oscillator tube. While thebias capacitor loa discharges rapidly, the delay circuit capacitor 24adischarges very slowly through the associated delay resistor 26a.Consequently, the negative bias which was developed across the delayresistor during the time when the tube of the irst oscillator A wasoperating is maintained for an additional period after the terminationof the triggering pulse at such a value that it pre- Vvents resumptionof current ilow and oscillation in the Vbias resistor and allows it tostart conducting and oscillation,

(2) They chargev retained on the delay capacitor 2d Y keeps thelirstoscillator tube cut off, and

(3) The initiation of oscillation in the second oscillator causes thebias voltage developed across the common grid bias resistor 29to returnto its original value which will maintain the lirst oscillator cut oiiafter the delay capacitor has discharged.

The circuit can be made toroperate without the delay Y networks if thecircuit constants and operating conditions are carefully chosen. This ispossible because a previously idle tube always has a smaller negativebias applied to its grid than a previously operating tube, andconsequently will recover lirst after the termination or" the negativetriggering pulse. However, the reliability of operation is enhanced bythe use of the delaying networks.

The voltage wave forms in Figure 2 are provided for a betterVunderstanding of the operation described above. Curve shows the negativetriggering pulses applied to the common grid bias resistor 29. Curves 42represent the bias voltages on the grids of the first and secondoscillator tubes.` The solid line wave form 4d represents the bias onthe grid of its rst oscillator tube grid. rEhe dotted line waveV form 46represents the bias on theY second oscillator tube grid. The dot-dashlines 43, 5b show the two.V critical bias leyelsreferred to above,namelyl the` cut-oli bias 48 and the oscillation-blocking-bias 50.

To insure that the delaying action of the delay networks is suiiicientto prevent the last operating oscillator from initiating oscillationafter the termination of a cutoff triggering pulse and beforeoscillation is initiated by the other oscillator, the time-constant ofthe discharge path of the delay capacitors should be made greater thanthe duration of the trigger pulse. However, if the oscillators are to berapidly alternated in operation, then the discharge time-constant of thedelay capacitor must be no larger than the time between triggeringpulses. it should be noted further that the discharge time-constant ofthe bias capacitors must be small as compared with the dischargetime-constant of the delay capacitors.

In an operative embodiment oi' the invention, the following values orcircuit elements were used:

Tubes lila and 10b are triode sections of a 12AU7 tube.

From the foregoing description, it will be readily ap-V parent thatthere has been described and shown herein a simple and inexpensive dualoscillator system, the individual oscillators of which may be switchedalternately into operation at a high speed and with reliability.

We claim:

l. A dual oscillator circuit having two stable states comprising, incombination, two oscillators, a separate biasing circuit for each ofsaid oscillatorsA adapted to develop a bias in response to the presenceof sustained oscillations, common biasing circuit means connected tosaid separate biasing circuits for applying Vthereto a part of saidoscillation-sustained bias such that the sustained oscillations ofoneroscillator prevents oscillation in the other, means for applyingtrigger pulses to said common biasing circuit of suicient magnitude tocut o all oscillations, and means including a bias delay network forretaining a part of said oscillation-sustained bias on a previouslyoperating oscillator after the cessation of its oscillation such that apreviously operating oscillator is prevented frombecoming operative fora predetermined period after the termination of a trigger pulse, wherebythe stable state of said dual oscillator circuit is alternate- Y lychanged in response to said trigger pulses.

2. A dual oscillator circuit as recited in claim l wherein each of saidoscillators includes an electron discharge tube having at least acathode, an anode, and a control grid, and a tuned tank circuit havingan inductor and a capacitor. Y 4

3. A dual oscillator circuit as recited in claim 2 wherein each of saidseparate biasing circuits for each oscillator comprises a Ybaiscapacitor whichY is connected between the grid of each tube and itsVassociated tank circuit, and a grid load resistor which is connected tothe grid of `said tube..

4. A dual oscillator circuit as recited in claim 3 wherein said commonbiasing circuit includes a common bias comprising a iirst and a secondelectron dischargevtube each having at least a cathode, an anode, acontrol grid, and a circuit connecting said plate to said control grid,means including a separate, tuned, tank circuit in said circuitconnecting said plate to said control grid, for determining thefrequency of oscillation through said tubes, separate biasing networksconnected to the grid of each tube each including a capacitor and a gridload resistor, a common bias resistor, a separate bias delay network foreach tube including a resistor and a capacitor connected in parallel,said delay network connecting said grid resistor to said common biasresistor, means for applying trigger pulses to said common biasingresistor of a magnitude and polarity such that oscillation is cut ot inboth tubes.

6. In combination, a iirst and a second electron discharge tube eachhaving at least an anode, a cathode, and a control grid, separate meansconnected between the anode and grid of each tube for establishingoscillations, a separate bias circuit connected to the grid of each tubeand adapted to develop a bias in response to the presence of sustainedoscillations, said separate biasing circuits each including a capacitorand a grid load resistor, a

common biasing circuit means connected to said separate biasing circuitsand adapted to apply a fraction of the bias from one of said separatebias circuits connected to the grid of one of said tubes which is inoscillation to the control grid of the other tube such that theoscillation through said one tube prevents conduction and oscillalationthrough the other, means for applying trigger pulses to said commonbiasing circuit of a magnitude and polarity such that conduction andoscillation is prevented in both tubes, a bias delay network for eachtube including a resistor and a capacitor connected in parallel, saidbias delay network being connected to said grid bias resistors and tosaid common biasing circuit.

References Cited in the iile of this patent UNITED STATES PATENTS2,150,241 Nichols Mar. 14, 1939 2,279,319 Honaman Apr. 14, 19422,445,448 Miller July 20, 1948 2,448,336 Weiner Aug. 31, 1948

